Polak187, I remember reading various comments about the Miox MSR somewhere a while ago and I just found the webpage. You might find some interesting information
here. Someone from Miox is actually answering people's questions there.
There is a specific comment regarding the Steri-Pen vs the MSR. On that webpage, just do a search for "Katie Bolek [12:51 PM March 31, 2005]" and you'll see the comments.
Basically, the Miox rep is saying that the Steri-Pen's one great advantage is speed, however the MSR is more flexible in terms of treating less-than-clear water and can reliably treat a wider variety of pathogens. I have an "older" Steri-Pen and batteries could be another Steri-Pen advantage (AA vs CR123a). Looks like the new Steri-Pens use CR123a's now, too, so no difference there.
The main problem people mention with the MSR is a "low salt" indicator light, which apparently is maily a design flaw with an early model. Doesn't really affect the operation, but is a false warning due to the brine solution getting trapped in one of the compartments (I've never handled one so I'm not quite sure "where" it gets trapped)
Someone does mention quickly discharged batteries, but that sounds like a defective unit (sucks when you discover the problem out in the field, though). Sounds like the low salt thing often surprised people because it worked fine at home.
Anyway, I think you'll find some of the info to be interesting.